1. Field of the Invention
The present invention relates to a ceramic product provided with multiple ceramic members. In particular, the present invention relates to a ceramic product provided with at least two ceramic members bonded to one another (examples include oxygen separation membrane elements and solid oxide fuel cells (SOFC)). The present invention also relates to a method for bonding together two ceramic members, and to a bond material for use in this method. In particular, the present invention relates to a method (seal method) and material (seal material) for bonding the solid electrolyte and separator making up an SOFC. The present invention also relates to a seal part (bond part) for maintaining the airtightness of the bond part of an oxygen separation membrane element provided with an oxygen separation membrane made of a perovskite oxide ceramic which is an oxygen ion conductor, as well as a bonding method (seal method) and bond material (seal material) for forming this seal part.
Priority for this application is claimed based on Japan Patent Application No. 2008-042443, submitted on Feb. 25, 2008, and Japan Patent Application No. 2008-042449, submitted on Feb. 25, 2008, and the entire content of these basic applications is incorporated by reference in these specifications.
2. Description of the Related Art
Ceramic products composed of multiple ceramic members bonded together are used in various industrial fields.
One example of such a ceramic product is an oxygen separation membrane element provided with a porous substrate supporting an oxygen separation membrane consisting of an oxide ceramic with a perovskite structure which is an oxygen ion conductor. A simple explanation of an oxygen separation membrane element is provided below.
Oxide ceramics with so-called perovskite structures and oxide ceramics with pyrochlore structures are known as oxygen ion conductors with oxygen ion (typically O2−; also called oxide ion) conductivity. In particular, dense ceramic materials consisting of perovskite oxide, a mixed oxygen ion-electron conductor (hereunder called a “mixed conductor”) that has electron conduction properties in addition to being an oxygen ion conductor, and typically ceramic materials formed as membranes, are capable of transmitting oxygen ions continuously from one side of the membrane to the other without the use of an external electrode or external circuit to short-circuit between the two sides. Consequently, they can be used favorably as oxygen separation materials for selectively transmitting to one side of a membrane oxygen from an oxygen-containing gas (such as air) supplied to the other side, particularly at high operating temperatures of 800 to 1000° C.
For example, an oxygen separation material (oxygen separation membrane element) provided with an oxygen separation membrane composed of a perovskite oxide or other mixed conductor on a porous substrate can be used favorably as an effective oxygen purification means in place of low-temperature separation or PSA (Pressure Swing Adsorption).
Alternatively, an oxygen separation membrane element of this type can also be used favorably in the field of fuel cells, or in GTL (Gas to Liquid) technology in which a synthetic liquid fuel (methanol or the like) is produced by using oxygen ions supplied from one side of the membrane to the other to oxidize a hydrocarbon (methane gas or the like) supplied to the other side.
As examples of such prior art, Japanese Patent Application Laid-open No. 2000-251534, Japanese Patent Application Laid-open No. 2000-251535, Japanese Patent Publication No. 2000-511507 and Japanese Patent Application Laid-open No. 2001-93325 describe various perovskite oxides that are mixed conductors. International Publication No. WO 2003/040058 pamphlet, Japanese Patent Application Laid-open No. 2006-82040, Japanese Patent Application Laid-open No. 2007-51032, Japanese Patent Application Laid-open No. 2007-51034 and Japanese Patent Application Laid-open No. 2007-51035 disclose good examples of oxygen separation materials (membrane elements) provided with oxygen separation membranes composed of perovskite oxides. Japanese Patent Application Laid-open No. H11-70314 and Japanese Patent Application Laid-open No. 2002-292234 describe cylindrical oxygen separation materials (elements) and devices (modules) provided with these oxygen separation materials.
When oxygen separation devices (modules) are constructed using such cylindrical or otherwise-shaped oxygen separation materials (membrane elements) as basic constituents, because various members are bonded together, the oxygen separation material is constructed with a seal part (bond part) for maintaining airtightness.
In the past, in the case of oxygen separation units (modules) which are used at high operating temperatures of 800 to 1000° C., glass materials and metal materials have been studied as seal materials for the seal part (bond part) in order to ensure the seal properties (airtightness) of the seal part. For example, Japanese Patent Application Laid-open No. 2002-83517 and Japanese Patent Application Laid-open No. 2002-349714 describe examples of conventional seal materials.
Other examples of ceramic products composed of multiple ceramic members bonded together include solid oxide fuel cells (SOFCs), which consist of a fuel electrode, an air electrode, a zirconia solid electrolyte and a separator bonded to this solid electrolyte.
Solid oxide fuel cells (SOFCs), which are also called solid electrolyte fuel cells, have been extensively developed for electrical power generation because they have greater generating efficiency than various other types of fuel cells, are environmentally friendly, and are applicable to various types of fuels.
The basic structure of an SOFC (that is, a single cell) consists of an air electrode (cathode) with a porous structure formed on one side of a dense solid electrode (such as a dense membrane layer) consisting of an oxide ion conductor, and a fuel electrode (anode) with a porous structure formed on the other side of the electrode. Fuel gas (typically hydrogen) is supplied to the surface of the solid electrolyte on the side with the fuel electrode and a gas containing oxygen (typically air) is supplied to the surface of the solid electrolyte on the side with the air electrode.
Since a single SOFC cell can only generate a limited amount of power, normally a stack of many such single-celled structures is used in order to obtain the desired electricity. In the case of stacked SOFCs, separators (also called interconnectors) are used to separate the cells. The separators then needed to be bonded (sealed) to the facing solid electrolyte surfaces so as to maintain a high degree of airtightness between the two.
Solid electrolytes consisting of zirconia materials (typically yttria-stabilized zirconia or YSZ) are widely used as solid electrolytes for SOFCs because of their chemical stability and mechanical strength. Cermets of Nio and zirconia for example are widely used for the fuel electrode, while LaCoO3, LaMnO3 and other oxides with perovskite structures are often used for the air electrode.
Because SOFCs normally have the temperature characteristic of operating well at high temperatures of about 800 to 1200° C., the material forming the separator is chosen so as to have high chemical durability and electrical conductivity in oxidizing and reducing atmospheres at high temperatures, and to have a similar thermal expansion rate as the electrolyte material. Examples of good separator-forming materials include lanthanum-chromite oxides (such as LaCrO3 and La0.8Ca0.2CrO3) and the like.
In the past bond materials of various kinds have been proposed for bonding (sealing) solid electrolytes and separators made of such materials. For example, Japanese Patent Application Laid-open No. H5-330935 describes a bond agent consisting of a mixture of stabilized zirconia and glass. Japanese Patent Application Laid-open No. H9-129251 describes a bond material consisting of a mixture of the solid electrolyte material and the separator material. Japanese Patent Application Laid-open No. H11-154525 describes a sealing liquid consisting mainly of an ultrafine particle oxide with a melting point greater than the operating temperature of a solid electrolyte fuel cell. Japanese Patent Application Laid-open No. H11-307118 is another example of prior art relating to bonding of solid electrolytes.